Tsinghua Zheng1, Matthew Finnerty1, Xiaoyu Yang1, Matthew Diprimio1, Luke Beery1, Paul Taylor1, Johanna Vannesjo2, Stuart Clare2, and Hiroyuki Fujita1,3,4,5
1Quality Electrodynamics, LLC, Mayfield Village, OH, United States, 2FMRIB Centre, Oxford University, Oxford, United Kingdom, 3Physics, Case Western Reserve University, Cleveland, OH, United States, 4Radiology, University Hospital of Cleveland, Cleveland, OH, United States, 55School of Information and Electrical Engineering, the University of Queensland, Brisbane, Australia
Synopsis
A cervical spine
array coil with a volume transmit coil for 7.0 Tesla was constructed and tested. The coil
uses one partially shielded birdcage volume transmit coil for generating
uniform excitation throughout the cervical spine region and an array of sixteen
loop coils for receiving. Initial volunteer imaging demonstrated good coverage
and uniformity along cervical spine.Introduction
For MRI imaging at
ultra-high frequency (UHF), it has always been challenging to generate a
uniform B1 transmit field at the desired region of interest. For cervical spine
imaging, this task is even more difficult due to the unique anatomical shape,
which limit the design options for the transmit coil design. In some previous studies
(1) (2), local transmit and receive surface coil arrays were used for
imaging the cervical spine at 7 Tesla. However,
due to the B1 characteristics of the surface coil, as well as lacking coil
elements from the anterior, it is quite difficult to generate a uniform
excitation for the entire imaging volume.
This is especially true for the lower spine region (C5-C7), where the
signal tends to degrade more quickly. In this study, we propose a birdcage coil
with shoulder cut-out as a volume transmitter, so that the B1 field generated
by the coil can reach the lower part of the cervical spine.
Method
Fig. 1 shows the layout
of the transmit coil and its relative location with the head and neck region – the shoulder cut-out on the birdcage
enables the patient’s neck to move further into the coil. A partial RF shield
which has been described in previous work (3) was used to improve
the efficiency of the coil. EM simulation was performed to ensure the shoulder cut
out on the birdcage did not significantly affect the B1 profile. Fig. 2 shows
the layout of the sixteen receive elements that were placed around the neck.
Isolation between adjacent coil elements are achieved by critical overlap, and
the coil elements are further isolated with each other by using an ultra-compact low
input impedance preamplifier (4). Mechanical design of the cervical
spine coil is shown in Fig. 3; it is a split top design with openings on the
anterior piece to facilitate patient positioning and comfort.
Results
The cervical spine
array coil was evaluated on a Siemens MAGNETOM 7.0 Tesla system. Images were
acquired with a healthy volunteer using GRE protocol (Axial, 0.5x0.5mm in-plane resolution, 3mm slice thickness, TR
500ms, TE 4ms, total acquisition time 2 minutes). Results showed that image quality for different
section of cervical spine (C4, 5 and 6 as shown in Fig. 4) are similar and the
signal uniformity for the cervical spine and surrounding tissue were quite
uniform. However, there is still visible signal difference between the upper
and lower spine area, which we would like to address in future study using
combination of volume Transmit coil and pTx technology.
Conclusion
A cervical spine
array coil for 7.0 Tesla was
constructed and tested. Using a partially shielded birdcage coil as volume
transmitter, we were able to get good coverage and uniformity along the cervical
spine for initial volunteer imaging.
Acknowledgements
No acknowledgement found.References
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JA. “High-resolution human cervical spinal cord imaging at 7 T”, NMR Biomed.
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Webb, “A radiofrequency coil
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(3)
M. Finnerty et al, “A 7-Tesla High Density Transmit with 28-Channel Receive-Only Array Knee
Coil”, ISMRM proceedings 2010 #642.
(4)
H. Fujita, et al., “A 3T
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